1. Field of the Invention
The present invention relates to an information recording medium usable as a personal authentication medium such as a license, credit card, or membership card, and a method of manufacturing the same.
2. Description of the Related Art
Various forgery/alteration preventing means are conventionally used in personal authentication media such as a license, credit card, and membership card. For example, a rainbow hologram that is a relief hologram having a metal reflecting layer is formed on the surface of a credit card as a structure for visually determining the authenticity of the card. Since this rainbow hologram formed on the card can optically be forged, demands have arisen for an advanced technique as a forgery preventing means.
Under the circumstances, Japanese Patent No. 3563774, for example, has disclosed an information recording medium in which a region having a first light-diffracting structure from which recorded information can mechanically be read and a region having a second light-diffracting structure from which recorded information can visually be read are alternately formed in a single light-diffracting structure formation layer so as not to superpose each other. When the first light-diffracting structure is a hologram from which recorded information can be read with only highly coherent light, it is visually difficult to perceive that optically readable information is recorded. This makes forgery difficult. Furthermore, when the second light-diffracting structure is a hologram from which recorded information can visually be read with white-light illumination, no special apparatus is necessary in visual authentication. Also, since the first and second light-diffracting structures do not superpose each other, no cloudiness occurs due to superposition of the two light-diffracting structures. Accordingly, operation errors hardly occur during mechanical reading. However, both the first and second light-diffracting structures contain permanent information, so the medium can easily be duplicated if the recorded information is revealed.
On the other hand, when a thin film of a polymer compound containing an azobenzene structure is directly irradiated with linearly polarized light, a photo-excited molecular reorientation phenomenon forms a thin birefringent film in which the molecular principal axis is oriented perpendicularly to the electric field vector of the irradiating light. This birefringence is attracting attention because it can be erased when irradiated with circularly polarized light and hence can be used as a rewritable hologram memory. This is described in, e.g., Opt. Commun., Vol. 47, (1983), p. 123 and Applied Physics Society Journal, Applied Physics, Vol. 75, No. 10, (2006), pp. 1,252-1,254. When the polymer compound containing an azobenzene structure is applied as a rewritable hologram to a forgery/alteration preventing means, variable information can be recorded instead of permanent information. This can make duplication difficult. Since information is rewritable, however, if visually readable information is rewritten, the information cannot visually be distinguished from authentic information. This makes the medium vulnerable to alteration.
Furthermore, when the application energy or application time of a laser beam is made higher or longer than that when forming the birefringence on the thin film of the polymer compound containing an azobenzene structure, a three-dimensional pattern is formed on the surface in accordance with the polarized state of the laser. As described in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2002-74665, this pattern can be erased by applying uniform light having a different wavelength or by heating. Accordingly, attempts have been made to apply a rewritable hologram to a high-density optical information recording method or the like.